Role of Fault Interactions in the Evolution of the Rio Grande Rift of North-Central New Mexico: Insights from Numerical Experiments

The late Cenozoic E-W extension in the Rio Grande Rift of north central New Mexico was predominantly accommodated by the en echelon N-S Pajarito and Taos faults and the intervening Embudo relay zone (ERZ). Traditional interpretations suggest that this segment of the rift evolved in two distinct episodes of irrotational plane strain in different orientations; they do not fully consider the role of mechanical interaction between the Pajarito and Taos faults, and the resulting complex strain fields. Also, vertical axis rotations observed in syn-rift basin fill suggest that this rift segment evolved under a combination of rotational and distortional strains.

Using the commercial finite element package ABAQUS, we have developed a displacement-based, three dimensional nonlinear numerical model to understand the role of mechanical interaction between en echelon faults in the evolution of the strain field in the intervening relay zone. Under an imposed orthogonal extension the interaction between the primary faults results in a combination of rotational and distortional strains within the relay zone. Maximum extension vectors within the zone initiate oblique to the imposed extension and rotate towards that direction with increasing extension.

Thus various oblique structures observed in the ERZ may have formed during the same episode of regional orthogonal extension that gave rise to the Pajarito and Taos faults. In addition, the localization of various secondary Miocene and Neogene faults, geographic variations in the vertical axis rotations from paleomagnetic studies, and field observations on the orientations of fracture networks in the ERZ are all consistent with near surface variations in the 3-D strain field predicted by the model. Therefore, within the ERZ, orientations of the structures alone do not necessarily warrant distinct deformation episodes involving rotation of far-field paleostresses.